1. Field of the Invention
The present invention relates to a magnetic recording type of photographic apparatus capable of effecting motion-image recording and still-image recording.
2. Description of the Related Art
In the field of magnetic recording, a demand for high-density recording has recently been increasing. To meet such a demand, there is provided, for example, a video tape recorder (VTR) of the type which is capable of effecting magnetic recording of high density by transporting a magnetic tape at a reduced speed. However, a VTR provided with a fixed head for recording an audio signal has the disadvantage that if an audio signal is recorded while simply transporting a magnetic tape at a reduced speed, no large relative speed is obtained and the quality of reproduced sound is degraded. For this reason, a method has been adopted in which the length of each track scanned by a rotary head is extended with respect to the track length used in the conventional art and an audio signal which is compressed along its time axis is sequentially recorded in the additional portion of each extended track.
By way of example, such a method will be explained with reference to a VTR of the rotary 2-head helical scan type. As shown in FIG. 1, a magnetic tape 1 is wrapped around a rotary cylinder 2 through an angle of (180+.theta.) degrees or more and, as shown in FIG. 2, a video signal recording area 5 and a PCM audio signal recording area 6 are traced by a rotary head 3 or 4 during the rotation of the rotary cylinder 2 through 180 degrees and during the rotation of the same through .theta. degrees, respectively. A PCM audio signal which is compressed along its time axis is recorded in the PCM audio signal recording area 6.
A method of recording a still image in a digital signal recording area has been proposed as an applied example of the aforesaid method of recording a digital signal in one area while recording a video signal in another area. Since the amount of information carried by a still image is comparatively small, it is possible to record the entire still-image information in the PCM audio signal recording area 6 on the magnetic tape 1 by scanning a number of PCM audio signal recording areas 6. According to this method, not only is it possible to realize still-image photography utilizing a photographic apparatus and a recording medium which are identical to those used for motion-image photography, but it is also possible to provide a high-quality still image. The quality of this still image is higher than that of a still image obtainable by repeatedly reproducing a video signal from the same track on a magnetic tape in a VTR while keeping the magnetic tape in a temporarily stopped state.
In such a magnetic recording type of photographic apparatus capable of recording a motion image and a still image, it is possible to perform automatic exposure control similar to that of a conventional camera for motion-image photography.
One example of automatic exposure control used in the conventional camera for motion-image photography will be described below with reference to FIG. 3. FIG. 3 shows an example of a camera for motion-image photography capable of performing automatic exposure control based solely on an automatic iris control.
Light which has passed through an optical system 601 is conducted to an exposure adjustment mechanism (iris) 602, and the amount of the light is adjusted by the iris 602. The light is then made incident on an image sensor 603 such as a CCD and is converted into an electrical signal by the image sensor 603. The electrical signal from the image sensor 603 is subjected to processing such as gamma correction and separated into a luminance signal and a chrominance signal, by a camera signal processing circuit 604. The luminance and chrominance signals are converted into a standardized television video signal which conforms to, for example, the NTSC system, by a camera encoder 605.
In the meantime, a luminance signal Y which has not been subjected to gamma correction in the camera signal processing circuit 604 is supplied to an integrator 606, where the luminance signal Y is subjected to integration processing. An arithmetic device 607 calculates the difference between the output of the integrator 606 and a reference value to generate a difference signal. The arithmetic device 607 supplies the difference signal to a driver 609 as an exposure control signal. The driver 609 causes the actuator 610 to control the aperture size of the iris 602 on the basis of the exposure control signal, thereby keeping constant the relationship between the output of the integrator 606 and the predetermined reference value.
Another example of automatic exposure control used in the conventional camera for motion-image photography will be described below with reference to FIG. 4. FIG. 4 shows an example of a camera for motion-image photography capable of performing automatic exposure control based on an automatic iris and automatic level control.
Light from a subject passes through a lens 702 and an iris 703 and is made incident on an image sensor 704, where the incident light is converted into an electrical signal. The iris 703 is controlled by the automatic iris circuit 705 on the basis of the electrical signal from the image sensor 704. In the meantime, the signal from the image sensor 704 is supplied to a voltage-controlled amplifier (VCA) 706, where the level of the signal is controlled. An automatic gain control (AGC) circuit which is a constituent element of the VCA 706 is controlled by an AGC control circuit 707 in accordance with the output of the VCA 706. A signal processing circuit 708 separates the output signal of the VCA 706 into a chrominance signal and a luminance signal and performs predetermined processing to output a standard video signal (according to the NTSC or PAL system).
Such a camera for motion-image photography is required to complete automatic exposure control in a short time since the state of a subject to be photographed varies temporally continuously during motion-image photography. However, if the speed of the automatic exposure control is excessively fast, the amount of exposure may exceed a desired amount exposure or an exposure control operation may be repetitively performed. As a result, the photographed subject may be reproduced as a continuous image of insufficient image quality. For this reason, it is desirable that the automatic exposure control be performed with smoothness rather than at a high speed.
On the other hand, still-image photography is achieved by freezing instantaneously the motion of a subject to be photographed. Accordingly, to prevent a shutter opportunity from being missed, high-speed automatic exposure and rapid control free from error are desired. As a result, if automatic exposure control similar to that used for the motion-image photography is performed during the still-image photography, no good exposure is achieved.
In the magnetic recording type of photographic apparatus as shown in FIG. 1, it is possible to utilize AF control similar to that of a motion-image photographic apparatus. Since the state of a subject to be photographed varies temporally continuously during motion-image photography, it is desired that the focusing time required for AF in the motion-image photographic apparatus be made short. However, if an object other than a subject being photographed passes across the scene, excessively fast AF will respond to the object abnormally sensitively, and the photographed subject may be reproduced as a continuous image of insufficient image quality. For this reason, it is desirable to take account of smoothness rather than high-speed response with respect to the performance of AF.
A so-called hill climbing system is known as one automatic focus adjusting method to meet the above-described demand. In the hill climbing system, a high-frequency component is extracted from a video signal obtained from an image sensor and a photographic lens is moved for focusing purpose until the level of the high-frequency component reaches its maximum.
An automatic focus adjusting method utilizing the above-described hill climbing system in the motion-image photographic apparatus shown in FIG. 5 will be described below.
Light passes sequentially through an F lens 101 for focus adjustment, a V lens 102 for magnification variation, a C lens 103 for effecting correction to hold a focus plane, an iris 104 and an RR lens 105 for correctly focusing the light on an image sensing plane. The light is focused on the image sensing plane of an image sensor 106 and converted into an electrical signal. The video signal outputted from the image sensor 106 is amplified to a predetermined level by a preamplifier 107, and is then converted into a standard television signal through predetermined processing such as gamma correction, blanking processing and addition of a sync signal, by a camera signal processing circuit 108.
The image sensor 106 is made to wobble along the optical axis to a slight extent in a predetermined cycle in synchronism with a timing signal generated from a timing generating circuit 114, and the image sensing plane is cyclically vibrated back and forth. A variation which occurs in the state of focus in accordance with the vibration is formed into a modulating signal and the sensed-image signal is modulated.
The video signal from the preamplifier 107 is also supplied to a band-pass filter (BPF) 109, where a high-frequency component which varies with the state of focus is extracted from the video signal. Then, in a gate circuit 110, only a signal portion corresponding to a focus detecting area (ranging frame) which is defined in a part of a viewfinder screen is extracted from the high-frequency component. Then, a peak value appearing during a frame period is detected by a peak detecting circuit 111, and the detected peak value is envelope-detected by a sync detecting circuit 112 in synchronism with a timing signal generated from the timing generating circuit 114. Since the image sensor 106 is made to wobble back and forth along the optical axis as described above, signals AN, AF and AM are applied to the peak detecting circuit 111 in response to the wobbling of the image sensor 106. The signals AN and AF appear on a near side and a far side, respectively, and are in reverse phase with each other, and the signal AM has an amplitude which reaches a minimum at an in-focus point. The signals are sync-detected by the sync detecting circuit 112 on the basis of the same frequency as the frequency of the wobbling, and an output B is obtained as shown in FIG. 6(b). As shown, the output B has a waveform whose signs on the respective near and far sides are reverse to each other and which crosses zero at the in-focus point.
FIG. 6(a) shows an output A obtained by plotting peak levels which are detected by the peak detecting circuit 111 as a lens is moved between a near point and a far point through the in-focus point. FIG. 6(b) shows the corresponding variation of the output B of the sync detecting circuit 112. The output A exhibits a hill-shaped characteristic curve which reaches a maximum at the in-focus point and becomes smaller toward each of the far and near sides.
The signal outputted from the sync detecting circuit 112 is appropriately amplified by an amplifier 113, and is then applied to a focusing-motor driving circuit 116. The focusing-motor driving circuit 116 determines whether the state of focus is near focus or far focus, and the F lens 101 is driven to move up to an in-focus point at a speed according to the envelope detection output. Then, a specific loop gain is set for the output of the sync detecting circuit 112 by the amplifier 113, and the focusing motor 117 is driven by the focusing-motor driving circuit 116 in accordance with the signal outputted from the amplifier 113, whereby the focus of the F lens 101 is adjusted.
However, a number of problems take place if the above-described automatic focus adjusting system is applied to the above-described magnetic recording type of photographic apparatus capable of recording a video signal in the recording area 5 and a digital still image signal in the recording area 6. For example, during still-image photography, it may take a long time to focus a subject to be photographed, or since hunting occurs during focusing, the subject may not be frozen instantaneously. As a result, a shutter opportunity will be missed.
The still-image photography in the above-described apparatus is achieved by freezing instantaneously a subject to be photographed. Accordingly, to prevent a shutter opportunity from being missed, a high-speed magnification varying operation is required and it is desired that an in-focus position be rapidly reached without error.
On the other hand, during the motion-image photography, the state of a subject to be photographed varies temporally continuously and it is, therefore, desired to reduce the time required to reach a target point during magnification adjustment. However, excessively fast adjustment may lead to the problem of exceeding the position of a desired focal length and hence the adjustment operation may be repetitively performed. As a result, the photographed subject may be reproduced as a continuous image of insufficient image quality. For this reason, it is desirable to take account of smoothness rather than high-speed response with respect to the performance of the magnification varying operation.
As is apparent from the above description, still-image photography and motion-image photography differ from each other with regard to the required nature of the magnification varying operation. If an identical magnification varying operation is carried out during each of the still-image photography and the motion-image photography, the problem that no appropriate adjustment of focal length is achieved will also take place.